1. Field of the Invention
The present invention relates to a device for controlling brakes of a vehicle such as an automobile, and more specifically, to such a device that executes follow-up (or servo) control of braking force or braking pressure for individual wheels of a vehicle.
2. Description of Prior Art
Electronic controlled braking (ECB) systems are being commonly employed in modern vehicles such as automobile, establishing braking force balance or distribution for providing an appropriate driving condition. In these ECB systems, braking force on individual wheels are servo-controlled. Namely, an electronic controller first determines target values of braking force or braking pressure (in a hydraulic system) for individual wheels, taking into account actual running conditions in the vehicle such as wheel speeds, the vehicle's behavior, as well as a braking action of the driver. Then, under the control of the controller, braking force generating apparatuses for the wheels are operated to generate braking force or pressures so as to follow up the respective target values. In order to bring actual braking force or pressure values for individual wheels in conformity with the corresponding target values, the actual values are fed back to the inputs of the respective braking force generating apparatuses so as to modify the inputs for canceling any deviation of the actual value from the corresponding target value, as well known in the art (feedback control). Examples of devices of ECB type are seen in Japanese Laid-Open Patent Publications (JP) Nos. 2000-247219 and 2002-187537. Such a system is often referred to as brake by wire (BBW) control system because a demand for controlling braking force is transmitted through electrical harness from an input device responding to braking action by a driver, e.g. a brake pedal, to braking force generating apparatuses for the respective wheels.
In practical ECB systems, braking force generating apparatuses and/or components in a hydraulic circuit of a braking system associated therewith, such as flow metering valves, have differences in input and output response characteristics within tolerances, hindering the accurate controlling of braking force on the individual wheels and/or braking force distribution. As mentioned above, a deviation of an actual braking force or pressure value for each wheel from the corresponding target value, if any, can be cancelled through the feedback control of braking force individually executed for each wheel. This feedback process, however, is not instantaneous. Until the actual value reaches to the target value, i.e. under transient condition, the rates of variation of the actual values can differ from one another among the wheels, dependent upon the response characteristics of the components in the braking system, resulting in undesirable difference of braking force or unwanted braking force distribution. In other words, in the controlling of braking force on individual wheels for an appropriate driving condition, the feedback strategy so far can not eliminate the effects of the tolerances under transient condition.
Accordingly, a braking force control device for a vehicle may be improved more appropriately for providing an appropriate driving condition, especially under transient condition (during the variation of braking force) in conjunction with the input-out response characteristics of braking force-generating apparatuses for wheels and/or components in a hydraulic braking circuit associated therewith.